Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
The image-receiving sheet for such a system is produced by coating, on a support, a dye-receptive layer and, when required, a layer having other functions, such as an intermediate layer. These layers may be produced by using solvent-based coating solutions or aqueous coating solutions.
After coating and drying of each of those layers, the image-receiving sheet is processed into the form of sheets or rolls according to specifications of a printer to be used. Then, the sheets or rolls are stored in appropriate packaging. The form of each packaging has no particular limitation, and one example thereof is the form of sack (case, pouch). As the material of each packaging, paper, plastic film or the like may be used. The thus packaged image-receiving sheets or rolls are transported and stored as they are or in a state that they are further contained in an appropriate casing, such as a corrugated-cardboard box.
A main purpose of packaging the image-receiving sheet is to prevent the sheet from deterioration of performance due to long-term storage. When there occur changes in performances such as sensitivity or transport ability for image-receiving sheets in a printer used, high-quality printed images cannot be obtained consistently. In fact, however, there are cases where it is difficult to keep the properties of image-receiving sheets stable for a long time. The long-term stability for the performance varies with properties of image-receiving sheets in themselves and manners of packaging. In particular, the long-term stability for the performance in the case of producing products obtained by using aqueous coating solution into the form of rolls and packaging them is inferior to that in other cases.
It is disclosed that changes in sensitivities of an ink sheet and an image-receiving sheet for use in a laser thermal transfer recording system can be suppressed by use of various packagings (e.g., JP-A-2000-141890 (“JP-A” means unexamined published Japanese patent application)). However, as a result of our examinations on thermal transfer systems, it has been found that there arises a new problem of causing changes in maximum transfer density. This problem arises only when image formation is carried out using an image-receiving sheet produced by using aqueous coating solutions, in which the sheet is processed into the form of rolls and packaged, and using a thermal head, not laser.